Introduce null-safety of Spring Framework API

This commit introduces 2 new @Nullable and @NonNullApi
annotations that leverage JSR 305 (dormant but available via
Findbugs jsr305 dependency and already used by libraries
like OkHttp) meta-annotations to specify explicitly
null-safety of Spring Framework parameters and return values.

In order to avoid adding too much annotations, the
default is set at package level with @NonNullApi and
@Nullable annotations are added when needed at parameter or
return value level. These annotations are intended to be used
on Spring Framework itself but also by other Spring projects.

@Nullable annotations have been introduced based on Javadoc
and search of patterns like "return null;". It is expected that
nullability of Spring Framework API will be polished with
complementary commits.

In practice, this will make the whole Spring Framework API
null-safe for Kotlin projects (when KT-10942 will be fixed)
since Kotlin will be able to leverage these annotations to
know if a parameter or a return value is nullable or not. But
this is also useful for Java developers as well since IntelliJ
IDEA, for example, also understands these annotations to
generate warnings when unsafe nullable usages are detected.

Issue: SPR-15540

spring-websocket/src/main/java/org/springframework/web/socket/CloseStatus.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.socket;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
@@ -177,6 +178,7 @@ public final class CloseStatus {
 	/**
 	 * Return the reason, or {@code null} if none.
 	 */
+	@Nullable
 	public String getReason() {
 		return this.reason;
 	}

spring-websocket/src/main/java/org/springframework/web/socket/WebSocketSession.java

@@ -25,6 +25,7 @@ import java.util.List;
 import java.util.Map;
 
 import org.springframework.http.HttpHeaders;
+import org.springframework.lang.Nullable;
 
 /**
  * A WebSocket session abstraction. Allows sending messages over a WebSocket
@@ -71,6 +72,7 @@ public interface WebSocketSession extends Closeable {
 	/**
 	 * Return the address on which the request was received.
 	 */
+	@Nullable
 	InetSocketAddress getLocalAddress();
 
 	/**
@@ -83,6 +85,7 @@ public interface WebSocketSession extends Closeable {
 	 * @return the protocol identifier, or {@code null} if no protocol
 	 * was specified or negotiated successfully
 	 */
+	@Nullable
 	String getAcceptedProtocol();
 
 	/**

spring-websocket/src/main/java/org/springframework/web/socket/adapter/NativeWebSocketSession.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.socket.adapter;
 
+import org.springframework.lang.Nullable;
 import org.springframework.web.socket.WebSocketSession;
 
 /**
@@ -31,6 +32,7 @@ public interface NativeWebSocketSession extends WebSocketSession {
 	 * Return the underlying native WebSocketSession, if available.
 	 * @return the native session or {@code null}
 	 */
+	@Nullable
 	Object getNativeSession();
 
 	/**
@@ -38,6 +40,7 @@ public interface NativeWebSocketSession extends WebSocketSession {
 	 * @param requiredType the required type of the session
 	 * @return the native session of the required type or {@code null}
 	 */
+	@Nullable
 	<T> T getNativeSession(Class<T> requiredType);
 
 }

spring-websocket/src/main/java/org/springframework/web/socket/adapter/jetty/JettyWebSocketSession.java

@@ -31,6 +31,7 @@ import org.eclipse.jetty.websocket.api.WebSocketException;
 import org.eclipse.jetty.websocket.api.extensions.ExtensionConfig;
 
 import org.springframework.http.HttpHeaders;
+import org.springframework.lang.Nullable;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.web.socket.BinaryMessage;
@@ -81,7 +82,7 @@ public class JettyWebSocketSession extends AbstractWebSocketSession<Session> {
 	 * @param user the user associated with the session; if {@code null} we'll fallback on the
 	 * user available via {@link org.eclipse.jetty.websocket.api.Session#getUpgradeRequest()}
 	 */
-	public JettyWebSocketSession(Map<String, Object> attributes, Principal user) {
+	public JettyWebSocketSession(Map<String, Object> attributes, @Nullable Principal user) {
 		super(attributes);
 		this.user = user;
 	}

spring-websocket/src/main/java/org/springframework/web/socket/adapter/jetty/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Adapter classes for the Jetty WebSocket API.
  */
+@NonNullApi
 package org.springframework.web.socket.adapter.jetty;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/adapter/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Classes adapting Spring's WebSocket API to and from WebSocket providers.
  */
+@NonNullApi
 package org.springframework.web.socket.adapter;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/adapter/standard/ConvertingEncoderDecoderSupport.java

@@ -17,6 +17,7 @@
 package org.springframework.web.socket.adapter.standard;
 
 import java.nio.ByteBuffer;
+
 import javax.websocket.DecodeException;
 import javax.websocket.Decoder;
 import javax.websocket.EncodeException;
@@ -32,6 +33,7 @@ import org.springframework.core.GenericTypeResolver;
 import org.springframework.core.convert.ConversionException;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.web.context.ContextLoader;
 
@@ -122,6 +124,7 @@ public abstract class ConvertingEncoderDecoderSupport<T, M> {
 	 * not using {@link ContextLoader}, this method should be overridden.
 	 * @return the {@link ApplicationContext} or {@code null}
 	 */
+	@Nullable
 	protected ApplicationContext getApplicationContext() {
 		return ContextLoader.getCurrentWebApplicationContext();
 	}

spring-websocket/src/main/java/org/springframework/web/socket/adapter/standard/StandardWebSocketSession.java

@@ -24,12 +24,14 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+
 import javax.websocket.CloseReason;
 import javax.websocket.CloseReason.CloseCodes;
 import javax.websocket.Extension;
 import javax.websocket.Session;
 
 import org.springframework.http.HttpHeaders;
+import org.springframework.lang.Nullable;
 import org.springframework.util.CollectionUtils;
 import org.springframework.web.socket.BinaryMessage;
 import org.springframework.web.socket.CloseStatus;
@@ -89,7 +91,7 @@ public class StandardWebSocketSession extends AbstractWebSocketSession<Session>
 	 * 	fallback on the user available in the underlying WebSocket session
 	 */
 	public StandardWebSocketSession(HttpHeaders headers, Map<String, Object> attributes,
-			InetSocketAddress localAddress, InetSocketAddress remoteAddress, Principal user) {
+			InetSocketAddress localAddress, InetSocketAddress remoteAddress, @Nullable Principal user) {
 
 		super(attributes);
 		headers = (headers != null) ? headers : new HttpHeaders();

spring-websocket/src/main/java/org/springframework/web/socket/adapter/standard/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Adapter classes for the standard Java WebSocket API.
  */
+@NonNullApi
 package org.springframework.web.socket.adapter.standard;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/client/jetty/JettyWebSocketClient.java

@@ -32,6 +32,7 @@ import org.springframework.core.task.AsyncListenableTaskExecutor;
 import org.springframework.core.task.SimpleAsyncTaskExecutor;
 import org.springframework.core.task.TaskExecutor;
 import org.springframework.http.HttpHeaders;
+import org.springframework.lang.Nullable;
 import org.springframework.util.concurrent.ListenableFuture;
 import org.springframework.util.concurrent.ListenableFutureTask;
 import org.springframework.web.socket.WebSocketExtension;
@@ -89,7 +90,7 @@ public class JettyWebSocketClient extends AbstractWebSocketClient implements Lif
 	 *
 	 * <p>By default an instance of {@code SimpleAsyncTaskExecutor} is used.
 	 */
-	public void setTaskExecutor(AsyncListenableTaskExecutor taskExecutor) {
+	public void setTaskExecutor(@Nullable AsyncListenableTaskExecutor taskExecutor) {
 		this.taskExecutor = taskExecutor;
 	}
 
@@ -192,6 +193,7 @@ public class JettyWebSocketClient extends AbstractWebSocketClient implements Lif
 	 * @return the user to make available through {@link WebSocketSession#getPrincipal()};
 	 * 	by default this method returns {@code null}
 	 */
+	@Nullable
 	protected Principal getUser() {
 		return null;
 	}

spring-websocket/src/main/java/org/springframework/web/socket/client/jetty/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Client-side support for the Jetty WebSocket API.
  */
+@NonNullApi
 package org.springframework.web.socket.client.jetty;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/client/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Client-side abstractions for WebSocket applications.
  */
+@NonNullApi
 package org.springframework.web.socket.client;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/client/standard/StandardWebSocketClient.java

@@ -26,6 +26,7 @@ import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.concurrent.Callable;
+
 import javax.websocket.ClientEndpointConfig;
 import javax.websocket.ClientEndpointConfig.Configurator;
 import javax.websocket.ContainerProvider;
@@ -38,6 +39,7 @@ import org.springframework.core.task.AsyncListenableTaskExecutor;
 import org.springframework.core.task.SimpleAsyncTaskExecutor;
 import org.springframework.core.task.TaskExecutor;
 import org.springframework.http.HttpHeaders;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.concurrent.ListenableFuture;
 import org.springframework.util.concurrent.ListenableFutureTask;
@@ -100,6 +102,7 @@ public class StandardWebSocketClient extends AbstractWebSocketClient {
 	/**
 	 * The configured user properties, or {@code null}.
 	 */
+	@Nullable
 	public Map<String, Object> getUserProperties() {
 		return this.userProperties;
 	}
@@ -110,7 +113,7 @@ public class StandardWebSocketClient extends AbstractWebSocketClient {
 	 * {@code doHandshake} methods will block until the connection is established.
 	 * <p>By default, an instance of {@code SimpleAsyncTaskExecutor} is used.
 	 */
-	public void setTaskExecutor(AsyncListenableTaskExecutor taskExecutor) {
+	public void setTaskExecutor(@Nullable AsyncListenableTaskExecutor taskExecutor) {
 		this.taskExecutor = taskExecutor;
 	}
 

spring-websocket/src/main/java/org/springframework/web/socket/client/standard/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Client-side classes for use with standard Java WebSocket endpoints.
  */
+@NonNullApi
 package org.springframework.web.socket.client.standard;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/config/MessageBrokerBeanDefinitionParser.java

@@ -38,6 +38,7 @@ import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.beans.factory.xml.BeanDefinitionParser;
 import org.springframework.beans.factory.xml.ParserContext;
 import org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean;
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.converter.ByteArrayMessageConverter;
 import org.springframework.messaging.converter.CompositeMessageConverter;
 import org.springframework.messaging.converter.DefaultContentTypeResolver;
@@ -260,6 +261,7 @@ class MessageBrokerBeanDefinitionParser implements BeanDefinitionParser {
 		return new RuntimeBeanReference(name);
 	}
 
+	@Nullable
 	private RootBeanDefinition getDefaultExecutorBeanDefinition(String channelName) {
 		if (channelName.equals("brokerChannel")) {
 			return null;
@@ -545,6 +547,7 @@ class MessageBrokerBeanDefinitionParser implements BeanDefinitionParser {
 		registerBeanDef(beanDef, context, source);
 	}
 
+	@Nullable
 	private RuntimeBeanReference getValidator(Element messageBrokerElement, Object source, ParserContext parserContext) {
 		if (messageBrokerElement.hasAttribute("validator")) {
 			return new RuntimeBeanReference(messageBrokerElement.getAttribute("validator"));

spring-websocket/src/main/java/org/springframework/web/socket/config/WebSocketMessageBrokerStats.java

@@ -25,6 +25,7 @@ import java.util.concurrent.TimeUnit;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.simp.stomp.StompBrokerRelayMessageHandler;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
@@ -75,6 +76,7 @@ public class WebSocketMessageBrokerStats {
 		this.stompSubProtocolHandler = initStompSubProtocolHandler();
 	}
 
+	@Nullable
 	private StompSubProtocolHandler initStompSubProtocolHandler() {
 		for (SubProtocolHandler handler : this.webSocketHandler.getProtocolHandlers()) {
 			if (handler instanceof StompSubProtocolHandler) {
@@ -105,6 +107,7 @@ public class WebSocketMessageBrokerStats {
 		this.loggingTask = initLoggingTask(1 * 60 * 1000);
 	}
 
+	@Nullable
 	private ScheduledFuture<?> initLoggingTask(long initialDelay) {
 		if (logger.isInfoEnabled() && this.loggingPeriod > 0) {
 			return this.sockJsTaskScheduler.scheduleAtFixedRate(new Runnable() {

spring-websocket/src/main/java/org/springframework/web/socket/config/WebSocketNamespaceUtils.java

@@ -27,6 +27,7 @@ import org.springframework.beans.factory.parsing.BeanComponentDefinition;
 import org.springframework.beans.factory.support.ManagedList;
 import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.beans.factory.xml.ParserContext;
+import org.springframework.lang.Nullable;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
 import org.springframework.util.StringUtils;
 import org.springframework.util.xml.DomUtils;
@@ -61,6 +62,7 @@ class WebSocketNamespaceUtils {
 		return handlerRef;
 	}
 
+	@Nullable
 	public static RuntimeBeanReference registerSockJsService(Element element, String schedulerName,
 			ParserContext context, Object source) {
 

spring-websocket/src/main/java/org/springframework/web/socket/config/annotation/AbstractWebSocketHandlerRegistration.java

@@ -20,6 +20,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
+import org.springframework.lang.Nullable;
 import org.springframework.scheduling.TaskScheduler;
 import org.springframework.util.Assert;
 import org.springframework.util.LinkedMultiValueMap;
@@ -140,6 +141,7 @@ public abstract class AbstractWebSocketHandlerRegistration<M> implements WebSock
 	 * if the application did not provide one. This should be done prior to
 	 * calling {@link #getMappings()}.
 	 */
+	@Nullable
 	protected SockJsServiceRegistration getSockJsServiceRegistration() {
 		return this.sockJsServiceRegistration;
 	}

spring-websocket/src/main/java/org/springframework/web/socket/config/annotation/WebMvcStompEndpointRegistry.java

@@ -22,6 +22,7 @@ import java.util.List;
 import java.util.Map;
 
 import org.springframework.context.ApplicationContext;
+import org.springframework.lang.Nullable;
 import org.springframework.scheduling.TaskScheduler;
 import org.springframework.util.Assert;
 import org.springframework.util.MultiValueMap;
@@ -146,6 +147,7 @@ public class WebMvcStompEndpointRegistry implements StompEndpointRegistry {
 	 * Return a handler mapping with the mapped ViewControllers; or {@code null}
 	 * in case of no registrations.
 	 */
+	@Nullable
 	public AbstractHandlerMapping getHandlerMapping() {
 		Map<String, Object> urlMap = new LinkedHashMap<>();
 		for (WebMvcStompWebSocketEndpointRegistration registration : this.registrations) {

spring-websocket/src/main/java/org/springframework/web/socket/config/annotation/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Support for annotation-based WebSocket setup in configuration classes.
  */
+@NonNullApi
 package org.springframework.web.socket.config.annotation;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/config/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Configuration support for WebSocket request handling.
  */
+@NonNullApi
 package org.springframework.web.socket.config;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/handler/package-info.java

@@ -2,4 +2,7 @@
  * Convenient {@link org.springframework.web.socket.WebSocketHandler}
  * implementations and decorators.
  */
+@NonNullApi
 package org.springframework.web.socket.handler;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/messaging/StompSubProtocolHandler.java

@@ -32,6 +32,7 @@ import org.apache.commons.logging.LogFactory;
 import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.context.ApplicationEventPublisherAware;
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.simp.SimpAttributes;
@@ -531,6 +532,7 @@ public class StompSubProtocolHandler implements SubProtocolHandler, ApplicationE
 		return connectedHeaders;
 	}
 
+	@Nullable
 	private String getDisconnectReceipt(SimpMessageHeaderAccessor simpHeaders) {
 		String name = StompHeaderAccessor.DISCONNECT_MESSAGE_HEADER;
 		Message<?> message = (Message<?>) simpHeaders.getHeader(name);

spring-websocket/src/main/java/org/springframework/web/socket/messaging/SubProtocolErrorHandler.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.socket.messaging;
 
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 
 /**
@@ -39,7 +40,8 @@ public interface SubProtocolErrorHandler<P> {
 	 * @return the error message to send to the client, or {@code null} in which
 	 * case no message will be sent.
 	 */
-	Message<P> handleClientMessageProcessingError(Message<P> clientMessage, Throwable ex);
+	@Nullable
+	Message<P> handleClientMessageProcessingError(@Nullable Message<P> clientMessage, Throwable ex);
 
 	/**
 	 * Handle errors sent from the server side to clients, e.g. errors from the
@@ -50,6 +52,7 @@ public interface SubProtocolErrorHandler<P> {
 	 * @return the error message to send to the client, or {@code null} in which
 	 * case no message will be sent.
 	 */
+	@Nullable
 	Message<P> handleErrorMessageToClient(Message<P> errorMessage);
 
 }

spring-websocket/src/main/java/org/springframework/web/socket/messaging/SubProtocolHandler.java

@@ -18,6 +18,7 @@ package org.springframework.web.socket.messaging;
 
 import java.util.List;
 
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.web.socket.CloseStatus;
@@ -67,6 +68,7 @@ public interface SubProtocolHandler {
 	 * Resolve the session id from the given message or return {@code null}.
 	 * @param message the message to resolve the session id from
 	 */
+	@Nullable
 	String resolveSessionId(Message<?> message);
 
 	/**

spring-websocket/src/main/java/org/springframework/web/socket/messaging/package-info.java

@@ -1,4 +1,7 @@
 /**
  * WebSocket integration for Spring's messaging module.
  */
+@NonNullApi
 package org.springframework.web.socket.messaging;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Common abstractions and Spring configuration support for WebSocket applications.
  */
+@NonNullApi
 package org.springframework.web.socket;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/server/RequestUpgradeStrategy.java

@@ -22,6 +22,7 @@ import java.util.Map;
 
 import org.springframework.http.server.ServerHttpRequest;
 import org.springframework.http.server.ServerHttpResponse;
+import org.springframework.lang.Nullable;
 import org.springframework.web.socket.WebSocketExtension;
 import org.springframework.web.socket.WebSocketHandler;
 
@@ -61,7 +62,7 @@ public interface RequestUpgradeStrategy {
 	 * handshake request.
 	 */
 	void upgrade(ServerHttpRequest request, ServerHttpResponse response,
-			String selectedProtocol, List<WebSocketExtension> selectedExtensions, Principal user,
+			@Nullable String selectedProtocol, List<WebSocketExtension> selectedExtensions, Principal user,
 			WebSocketHandler wsHandler, Map<String, Object> attributes) throws HandshakeFailureException;
 
 }

spring-websocket/src/main/java/org/springframework/web/socket/server/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Server-side abstractions for WebSocket interactions.
  */
+@NonNullApi
 package org.springframework.web.socket.server;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/server/standard/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Server-side classes for use with standard JSR-356 WebSocket endpoints.
  */
+@NonNullApi
 package org.springframework.web.socket.server.standard;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/server/support/AbstractHandshakeHandler.java

@@ -33,6 +33,7 @@ import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.server.ServerHttpRequest;
 import org.springframework.http.server.ServerHttpResponse;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
@@ -340,6 +341,7 @@ public abstract class AbstractHandshakeHandler implements HandshakeHandler, Life
 	 * @return the selected protocols or {@code null}
 	 * @see #determineHandlerSupportedProtocols(WebSocketHandler)
 	 */
+	@Nullable
 	protected String selectProtocol(List<String> requestedProtocols, WebSocketHandler webSocketHandler) {
 		if (requestedProtocols != null) {
 			List<String> handlerProtocols = determineHandlerSupportedProtocols(webSocketHandler);
@@ -402,6 +404,7 @@ public abstract class AbstractHandshakeHandler implements HandshakeHandler, Life
 	 * @param attributes handshake attributes to pass to the WebSocket session
 	 * @return the user for the WebSocket session, or {@code null} if not available
 	 */
+	@Nullable
 	protected Principal determineUser(ServerHttpRequest request, WebSocketHandler wsHandler,
 			Map<String, Object> attributes) {
 

spring-websocket/src/main/java/org/springframework/web/socket/server/support/HttpSessionHandshakeInterceptor.java

@@ -25,6 +25,7 @@ import javax.servlet.http.HttpSession;
 import org.springframework.http.server.ServerHttpRequest;
 import org.springframework.http.server.ServerHttpResponse;
 import org.springframework.http.server.ServletServerHttpRequest;
+import org.springframework.lang.Nullable;
 import org.springframework.web.socket.WebSocketHandler;
 import org.springframework.web.socket.WebSocketSession;
 import org.springframework.web.socket.server.HandshakeInterceptor;
@@ -160,6 +161,7 @@ public class HttpSessionHandshakeInterceptor implements HandshakeInterceptor {
 		return true;
 	}
 
+	@Nullable
 	private HttpSession getSession(ServerHttpRequest request) {
 		if (request instanceof ServletServerHttpRequest) {
 			ServletServerHttpRequest serverRequest = (ServletServerHttpRequest) request;

spring-websocket/src/main/java/org/springframework/web/socket/server/support/package-info.java

@@ -2,4 +2,7 @@
  * Server-side support classes including container-specific strategies
  * for upgrading a request.
  */
+@NonNullApi
 package org.springframework.web.socket.server.support;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/client/SockJsClient.java

@@ -30,6 +30,7 @@ import org.apache.commons.logging.LogFactory;
 
 import org.springframework.context.Lifecycle;
 import org.springframework.http.HttpHeaders;
+import org.springframework.lang.Nullable;
 import org.springframework.scheduling.TaskScheduler;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
@@ -323,6 +324,7 @@ public class SockJsClient implements WebSocketClient, Lifecycle {
 	 * <p>By default this method returns {@code null}.
 	 * @return the user to associate with the session (possibly {@code null})
 	 */
+	@Nullable
 	protected Principal getUser() {
 		return null;
 	}

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/client/TransportRequest.java

@@ -20,6 +20,7 @@ import java.net.URI;
 import java.security.Principal;
 
 import org.springframework.http.HttpHeaders;
+import org.springframework.lang.Nullable;
 import org.springframework.web.socket.sockjs.frame.SockJsMessageCodec;
 
 /**
@@ -62,6 +63,7 @@ public interface TransportRequest {
 	/**
 	 * Return the user associated with the request, if any.
 	 */
+	@Nullable
 	Principal getUser();
 
 	/**

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/client/package-info.java

@@ -2,4 +2,7 @@
  * SockJS client implementation of
  * {@link org.springframework.web.socket.client.WebSocketClient}.
  */
+@NonNullApi
 package org.springframework.web.socket.sockjs.client;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/frame/SockJsFrame.java

@@ -19,6 +19,7 @@ package org.springframework.web.socket.sockjs.frame;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -104,6 +105,7 @@ public class SockJsFrame {
 	 * for SockJS "open" and "close" frames, which do not contain data, return
 	 * {@code null}.
 	 */
+	@Nullable
 	public String getFrameData() {
 		if (getType() == SockJsFrameType.OPEN || getType() == SockJsFrameType.HEARTBEAT) {
 			return null;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/frame/SockJsMessageCodec.java

@@ -19,6 +19,8 @@ package org.springframework.web.socket.sockjs.frame;
 import java.io.IOException;
 import java.io.InputStream;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Encode and decode messages to and from a SockJS message frame,
  * essentially an array of JSON-encoded messages. For example:
@@ -48,6 +50,7 @@ public interface SockJsMessageCodec {
 	 * @return an array of messages, or {@code null} if none
 	 * @throws IOException if the content could not be parsed
 	 */
+	@Nullable
 	String[] decode(String content) throws IOException;
 
 	/**
@@ -56,6 +59,7 @@ public interface SockJsMessageCodec {
 	 * @return an array of messages, or {@code null} if none
 	 * @throws IOException if the content could not be parsed
 	 */
+	@Nullable
 	String[] decodeInputStream(InputStream content) throws IOException;
 
 }

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/frame/package-info.java

@@ -2,4 +2,7 @@
  * Support classes for creating SockJS frames including the encoding and decoding
  * of SockJS message frames.
  */
+@NonNullApi
 package org.springframework.web.socket.sockjs.frame;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Top-level SockJS types.
  */
+@NonNullApi
 package org.springframework.web.socket.sockjs;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/support/package-info.java

@@ -3,4 +3,7 @@
  * {@link org.springframework.web.socket.sockjs.support.AbstractSockJsService}
  * implementation.
  */
+@NonNullApi
 package org.springframework.web.socket.sockjs.support;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/handler/AbstractHttpSendingTransportHandler.java

@@ -23,6 +23,7 @@ import java.util.regex.Pattern;
 import org.springframework.http.MediaType;
 import org.springframework.http.server.ServerHttpRequest;
 import org.springframework.http.server.ServerHttpResponse;
+import org.springframework.lang.Nullable;
 import org.springframework.util.MultiValueMap;
 import org.springframework.util.StringUtils;
 import org.springframework.web.socket.WebSocketHandler;
@@ -112,6 +113,7 @@ public abstract class AbstractHttpSendingTransportHandler extends AbstractTransp
 	protected abstract SockJsFrameFormat getFrameFormat(ServerHttpRequest request);
 
 
+	@Nullable
 	protected final String getCallbackParam(ServerHttpRequest request) {
 		String query = request.getURI().getQuery();
 		MultiValueMap<String, String> params = UriComponentsBuilder.newInstance().query(query).build().getQueryParams();

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/handler/package-info.java

@@ -3,4 +3,7 @@
  * implementation classes as well as a concrete
  * {@link org.springframework.web.socket.sockjs.SockJsService}.
  */
+@NonNullApi
 package org.springframework.web.socket.sockjs.transport.handler;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/package-info.java

@@ -6,4 +6,7 @@
  * counterparts for sending messages over the various transports, and
  * {@link org.springframework.web.socket.sockjs.transport.handler.DefaultSockJsService}.
  */
+@NonNullApi
 package org.springframework.web.socket.sockjs.transport;
+
+import org.springframework.lang.NonNullApi;

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/session/package-info.java

@@ -2,4 +2,7 @@
  * SockJS specific implementations of
  * {@link org.springframework.web.socket.WebSocketSession}.
  */
+@NonNullApi
 package org.springframework.web.socket.sockjs.transport.session;
+
+import org.springframework.lang.NonNullApi;